Field of the Invention
The present invention relates to an imaging element.
Description of Related Art
In a general color digital camera, a single-plate imaging element configured with a light receiving element that is two dimensionally arranged thereon is adapted. In such a single-plate color digital camera, a color filter is disposed in font of the light receiving element to obtain a full color image. Types of color filters include an RGB primary color system and a CMYG complementary color system.
Hereinafter, a signal processing method will be described that generates a full color image in which all pixels have RGB color information in an imaging element which a color filter of the primary color system is mounted and in an imaging element on which a color filter of the complementary color system is mounted.
(Imaging Element on Which the Color Filter of the Primary Color System is Mounted)
FIG. 13 is a schematic view illustrating a color arrangement of the color filter of the primary color system, which is referred to as a Bayer array known in the related art. In the illustrated example, a color filter R for transmitting red (R) light, a color filter G for transmitting green (G) light, and a color filter B for transmitting blue (B) light are regularly arranged on a light receiving surface of each pixel. The color filter G accounts for ½ of the total number of pixels. In addition, the color filter R accounts for ¼ of the total number of pixels. Moreover, the color filter B accounts for ¼ of the total number of pixels.
As shown in FIG. 13, since any one of the color filter R, the color filter U and the color filter B is disposed on the light receiving surface of each pixel, the color information of two colors among red, green, and blue is always missing. Therefore, the missing information of the two colors is interpolated based on an output of an adjacent pixel, and demosaicing processing is performed so that each pixel obtains the color information of the three colors of red, blue, and green.
As the demosaicing, processing, two methods are generally known. The first method is a method of performing linear interpolation from an adjacent or nearby pixel.
The second method is a method of calculating a color difference signal after the linear interpolation, determining whether a correlation in a vertical direction or a horizontal direction is stronger, and further performing interpolation processing based on a result of the determination. For example, when the correlation in the vertical direction is stronger, interpolation is further performed with an average value of adjacent pixels in the vertical direction.
(Imaging Element on Which the Color Filter of the Complementary Color System is Mounted)
FIG. 14 is a schematic view illustrating a color arrangement of the color filter of the complementary color system, which is referred to as a color difference sequential system known in the related art. In an illustrated example, a color filter C for transmitting cyan light C, a color filter M for transmitting magenta light M, a color filter Y for transmitting yellow light Y, and a color filter G for transmitting green light G are regularly arranged on the light receiving surface of each pixel. Cyan (C) is the complementary color (=Blue (B)+Green (G)) of red (R). Magenta (M) is the complementary color (=Blue (B)+Red (R)) of green (G). Yellow (Y) is the complementary color (=Green (G)+Red (R)) of blue (B).
FIG. 15 is a graph which shows transmission characteristics of the color filter C, the color filter M, the color filter Y and the color filter G known in the related art. The horizontal axis of the graph shows a wavelength λ (nm). The vertical axis of the graph shows relative spectral sensitivity. As shown in FIG. 15, a curve 2501 shows transmission characteristics of the color filter C. A curve 2502 shows transmission characteristics of the color filter M. A curve 2503 shows transmission characteristics of the color filter Y. A curve 2504 shows transmission characteristics of the color filter G
Hereinafter, description will return to FIG. 14 As shown in FIG. 14, since any one of the color filter C, the color filter M, the color filter Y, and the color filter G is disposed on the light receiving surface of each pixel, the color information of three colors among cyan, magenta, yellow, and green is always missing in each pixel. In the complementary color system, conversion is performed in the following manner (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2004-56640) to obtain an RGB image to be displayed on a color television from a captured CMYG image.
First, the color information of all of CMYG in each pixel is calculated using the CMYG color information obtained one by one by each pixel. In a calculation method, linear interpolation and the like are performed. Then, the CMYG color information is converted into the RGB color information of each pixel. The transmittance of each color component of RGB transmitted through each color filter of CMYG is a parameter determined by the transmission characteristics of CMYG during manufacture.
FIG. 16 is a schematic view illustrating an example of the transmittance of each color component of RGB transmitting each color filter of CMYG known in the related art.
In the illustrated example, the transmittance of red light transmitted through the color filter G is 0.11, the transmittance of green light transmitted through the color filter G is 0.86, and the transmittance of blue light transmitted through the color filter G is 0.08. In addition, the transmittance of red light transmitted through the color filter M is 0.50, the transmittance of green light transmitted through the color filter M is 0.29, and the transmittance of blue light transmitted through the color filter M is 0.51. Moreover, the transmittance of red light transmitted through the color filter C is 0.11, the transmittance of green light transmitted through the color filter C is 0.92, and the transmittance of blue light transmitted through the color filter C is 0.75. Furthermore, the transmittance of red light transmitted through the color filter Y is 0.81 the transmittance of green light transmitted through the color filter Y is 0.98, and the transmittance of blue light transmitted through the color filter Y is 0.08.
When the transmittance of each color component of RGB transmitted through each color filter of CMYG is as shown in FIG. 16, color information conversion from CMYG into RGB in each pixel can be performed using a matrix operation (color matrix conversion) shown in the following Equation (1).
                              (                                                    R                                                                    G                                                                    B                                              )                =                              (                                                                                -                    0.61                                                                    0.88                                                                      -                    0.62                                                                    0.86                                                                              0.79                                                                      -                    0.60                                                                    0.30                                                  0.22                                                                                                  -                    0.54                                                                    0.89                                                  0.85                                                                      -                    0.59                                                                        )                    ⁢                      (                                                            G                                                                              M                                                                              C                                                                              Y                                                      )                                              (        1        )            
It is possible to perform conversion from the CMYG color information into the RGB color information by performing the above-described operation.